What's a Reverse Polish notation? It's a mathematical notation where the operator (e.g. +, -, x) follows the operands. Those of you who have the old HP-10C series calculator are probably nodding in nostalgia right now!

@Margaryna, RPN can't be a single variable unless you're claiming very non-standard syntactic rules. Spaces are used as token delimiters, meaning "reverse", "polish", and "notation" are all separate tokens. For RPN to be a since variable, it would have to be written as "RPN" or "reverse-polish-notation" or use some other connectors to maintain it as a single token. To claim its a single variable in its current form is to imply context variable syntax rules for the language, which is not normal.